Motor control system



Filed Jan. 9, 1930 Inventor: Carl .Schiebeler, Arno Weiss by Ma TheirAttorney Patented July 25, 1933 UNITED srarss arser orr ce CARLSCI-IIEBELER, OF BERLIN-HALENSEE, AND Ali-N0 WEISS, OFBERLIN-OHARLOTTEN- BURG, GERMANY, ASSIGNORS TO GENERAL ELECTRICCOIVIPANY, A CORPORATION OF NEW YORK MOTOR CDNTROIi SYSTEM Applicationfiled January 9, 1930, Serial No. 419,650, and in Germany Maya, 1929.

This invention relates to systems for controlling the operation ofelectric motors, more particularly to systems in which series woundmotors are required to start heavy loads from rest; and has for anobject the provision of a system for limiting the starting torque of themotor to a maxi mum permissible value and for causing the motor todeliver a series of impulses to the load in the event that the load doesnot start to move when the applied torque reaches its maximumpermissible value.

The invention, although generally ap plicable to systems in which anelectric motor is required to start heavy loads is particularlyadvantageous in its application to systems in which a series motor isemployed to drive a pipe foundry crane. It is well known that unusuallylarge pullin forces are required to Withdraw the core fiom the pipeafter it has been cast and because of the large torque that a serieswound motor is able to exert, when starting under load, motors of thistype are usually employed in this connection.

Furthermore it frequently happens that despite the provision ofintermediate layers of material designed to prevent their adhesion, thecore and the casting bake together and the motor is thus unable to startthe load immediately, in which event if a series motor is employed thetorque will increase to a point at which the mechanical parts of thecrane are endangered. In practice cases have occurred in which thetorque developed by series motors employed in driving pipe foundrycranes has resulted in damage and even in destruction to the craneitself. 7 V

In the operation of a pipe foundry crane the operator returnsthecontroller handle to the neutral position immediately he observesthat the core has not started to move when in his judgment the motortorque has reached a value that may not be exceeded without endangeringthe mechanical parts of the crane and the windings of the motor. Inorder to withdraw the core from the cast pipe the operator then switchesthe controller on and off to cause the motor to deliver a series ofimpulses to the load until the core is loosened in the casting afterwhich the core may easily be withdrawn therefrom. Our invention providesa solution for the above problem.

In carryingthis invention into effect in one form thereof, we providemeans for lime iting the starting torque of a drive motor to amaximumpermissible value together with 7 means for automatically causingthe motor to deliver a series of impulses to the load that does notstart to move when the maximum permissible starting torque is applied.

For better and more complete understanding of'the invention,reference'should now be had to the following specification andto theaccompanying drawing thesingle figure of which is a diagrammatical viewof the embodiment of the invention.

Referring now to the drawing, an electric motor 10 is mechanicallycoupled to drive a suitable load such for example as a crane employedfor the purpose of withdrawing cores from pipes after they have beencast. As shown the motor 10 is a series motor, that is to say it isprovided with a field winding 11 which is connected. in series with thearmature of the motor and thus conducts the entire armature current. Theuse of-a series wound motor is very advantageous in starting heavy loadssince its torque increases with increasing armature current. The motor10 is supplied with electrical energy from a suitable source of supplysuch for exampleas the generator 12 to which it is connected in WardLeonard fashion'by means of'conductors 13 and 14:; thegenerator 12itself being driven preferably ata substantially constant speed by anysuitable driving means such ternatin'g current electric motor 27 whichis supplied from a suitable source of electric power represented in thedrawing by the three supply conductors 28. As shown in the drawing thegenerator 12 is provided with a series field winding 15, a shunt fieldwinding 16 and a separately excited field winding 17 the separatelyexcited field winding 17 being energized from any suitable directcurrent for example as the alsource such as that represented in thedrawing by the two supply conductors 18.

The shunt field winding 16 and the separately excited field winding 17are cumulative, that is to say, they are so wound and energized that themagnetic fields resulting from their energization are in the samedirection and add to each other. The series field winding 15 on theother hand acts differentially with respect to the shunt field windingand the separately excited field winding 17, i. e., it is wound andexcited in such a manner that the magnetic field resulting from itsenergization opposes the magnetic fields due to the field windings 16and 17 Thus, the resultant or effective field of the generator 12 is thealgebraic sum of the magnetic fields resulting from the field windings16 and 17 diminished by the field produced by the field winding 15. Thenumber of turns of the series field winding 15 is so chosen that thecurrent which the generator 12 supplies to the motor 10 will be limitedto a predetermined value which value will be low enough to prevent thecurrent from doing damage to the windings of the motor and which willlimit the torque applied by the motor to a value that will prevent themotor from doing damage to the mechanical parts of the crane in case thecore does not start to move.

The strength of the separately excited field 17 of the generator isregulated by means of a regulating resistance 19 and a cooperatingmovable contact 20, the position of which on the resistor 19 isvaried asdesired by means of any suitable controlling means'such for example as amanually operated controller. An additional control resistance 21 isconnected in the circuit of the separately excited fieldwinding 17 whichis arranged to be short circuited by the contacts of an auxiliary relay22 when the re lay is energized. The ohmic value of the resistance 21 isso chosen that when the contacts 'ofthe relay 22 are open and it is incircuit with the field winding 17 the re sultant field of the generator,i. e., the algebraic sum of the magnetic fields produced by the fieldwindings 15, 16 and 17 when field winding 15 is energized to the maximumpermissible value is practically reduced to zero thereby causing thecurrent supplied by generator 12 to motor 10 to decrease to a negligiblevalue.

As shown, the auxiliary relay 22 is provided with an energizing windingwhich is supplied from the direct current source 18, and this relayserves when energized to bridge the contacts 23 to complete a shortcircuit about the control resistance 21 and when deenergized to open thecontacts 23 and thereby remove the short circuit about the resistance21,

The peration of t e o t el y .22 to insert and exclude .the resistance21 from the circuit of the field winding 17 is controlled by means of acontrol relay 24, the energizing winding of which is connected incircuit with the conductor 14 in the supply circuit between thegenerator 12 and the motor 10. When the control relay 24 is deenergizedits armature member bridges the contacts 25 in the circuit of theenergizing coil of the control relay 22 to complete the energization ofthe latter whilst when the relay 24 is energized its armature isattracted to the open position to open circuit the contacts 25 andthereby deenergize the coil of auxiliary relay 22. The coil of thecontrol relay 24 is so designed that it attracts its armature member tothe open position when current of the maximum permissible value abovedescribed traverses its winding and retains its armature member in theattracted position until the current flowing in its winding is reducedpractically to zero. A time delay device may be provided in conjunctionwith the control relay 24 to control the periodicity of its operation asdesired. As thus. constructed the control relay 24 serves to control theauxiliary relay 22 to insert and exclude the control resistance 21 fromthe circuit of the separately excited field winding 17 in response tothe current supplied by the generator 12 to the motor 10.

lVith the above understanding of the elements comprising the inventionand their organization, the operation of the system itself will bereadily understood from the de tailed description which follows:

When the movable contact 20 and the relay 22 are in the positions inwhich they are shown in the drawing, i. e., with the entire regulatingresistance 19, and the control resistance 21 in the circuit with thesepa-. rately excited field winding 17 the generator is practicallydeenergized and supplies no current to the motor 10. The disconnectswitch 26 is first closed to complete the energizing circuit for relay22 which then closes and bridges the contacts 23 to short circuit thecontrol resistance 21. Operation of the manually operated controller tovary the position of themovable contact 20 on the regulating resistance19, i. e., to decrease the resistance causes. the current flowing in thefield winding 17 to increase and thus causes the generator 12 to supplya currentto the motor 10 the value of which current increases inproportion to the increase of the current of the field winding17 as willbe well understood. In addition the voltage generated by the generator12 causes a cur rent to flow in the shunt field winding 16 whichproduces a field that adds to the field produced by the winding 17.However, as the current which the generator 12 supplies to the motor 10increases in value the magnetic field produced by the series fieldwinding 15 also increasesin value, and opposes orsubtracts from the manetic field due to the windings 16 and 17. hen the current supplied tothe motor 10 increases to the predetermined value above mentioned, i}e.,the value at which the current will do damage to the windings of themotor 10 or cause the motor 10 to do damage to the crane in the eventthat the core does not start to move, the subtractive efiect of thefields due to the series windings 15 is such that the current suppliedby the generator 12' cannot 'increase further. Thus, it will be seenthat the field windings '15, 16 and 17 are so designed and related toeach other that the output current of the generator 12 is limited to amaximum permissible value.

WVhen the current supplied by the generator 12 to the motor 10 reachesthis predetermined maximum permissible value the control relay 24attracts its armature out of engagement with the contacts '25 therebyopening the energizing circuit of the auxiliar rela 22. Deener izationof auxiliar y y b relay 22 permits its armature member to drop out ofengagement with the contacts;23 thereby removing the short circuit aboutthe resistor 21 and inserting it in circuit with the separately excitedfield winding 17.

As a result of the insertion of resistance 21 in the circuit of theseparately excited field winding 17 the magnetic field due to the fieldwindings 16 and 17 is practically overcome by the opposing magneticfield due to the series field winding 15 and thus, the resultantenergization of the generator 12 is practically reduced to zero. As aresult of the deenergization of the field of the generator the currentsupplied to the motor 10 decreases to a practically negligible valuethereby resulting in a deenergization of the control relay 24.

Due to the deenergization of the control relay its armature is permittedto descend under the attraction of gravity and again to bridge thecontacts 25 to complete the energizing circuit of auxiliary relay 22.Auxiliary relay 22 in responding to the energization of its coiloperates its armature to the upper or closed position to bridge thecontacts 23 and again complete a short circuit about the controlresistance 21 thereby excluding it from the circuit of the separatelyexcited field winding 17, the magnetic field of which then builds up toits former value thus causing the resultant field of the generator 12again to build up to a value sufiicient to supply current of maximumpermissible value to the motor 10.

If the motor torque and current again reach their maximum permissiblevalues before the core starts to move, the control relay 2 1 and theauxiliary relay 22 will again function in the manner described todecrease the current and torque of the motor to a minimum value andsubsequently to rapidly increase them totheir maximum permissiblevalues, thereby causing the motors to deliver a short sharp impulse tothe load. It will be clear that if the load does not move that the abovedescribed operation of decreasing and subsequently increasing the torqueand current of the motor will here peated' at regular intervals, thus ineffect causing the motor 10 to deliver to the load a series of torqueimpulses of maximum value, that continues until the core is loosened. Assoon, however, as the core becomes loosened in the casting andthe motor10 starts to rotate, the current taken by the motor to produce thetorque necessary to withdraw the core from the casting will actuallydecrease. Consequently, the current drawn by the motor 10 will not reachits maximum permissible value again"during that operation, and thus thecontrol relay 24 will remain unattracted in the position shown,bridgingthe contacts 25' and maintaining the auxiliary relay 22 in itsoperated position short circuiting the control resistmice 21 fortheremainder ofthe Withdrawal operation. i i

"From the foregoing description it will be clear "th'at'the operation ofthe system to limit the torque applied by the motor to the loadand-tocause the motor'to delivena series of impulses to the load in-the eventthat it does not start when the current and torque reach their maximumpermissible values is entirely automatic; no action being required ofthe operator other than to move the controller handle to an operativeposition to initiate the operation.

0 Although in accordance with the provisions of the patent statutes wehave described our invention as embodied in concrete apparatus organizedin a particular manner, it is to be understood that the embodiment shownis merely illustrative and that the fications and alterations willreadily suggest themselves to persons skilled in the art withoutdeparting from the true spirit of the invention or from the scope of theannexed claims.

What we claim as new and desire to se- 1. In a variable voltage controlsystem, a generator provided with a field circuit, a motor suppliedtherefrom for driving a load body, a resistance connected in said fieldcircuit, and means responsive to the current exchanged between saidgenerator and said motor for intermittently short circuiting saidresistance to produce variations in the current supplied to said motorbetween a maximum value and a substantially lower minimum value therebyto cause said motor cure by Letters Patent of the United States 11Dinvention is not limited thereto since modito deliver a Series ofdriving impulses to said load. a I

2. Ina variable voltage control system a generator provided with aseparately excited field winding a-ndan oppositely Wound series fieldWinding for limiting the armature current of said generator to beaqpredetermined value, a motor supplied from said generator for drivinga load, a normally short circuited resistance in said separately excitedfield circuit, means for increasing the currentsupplied to said motor tosaid pres determined value, and means responsive to said predeterminedvalue of current to interrupt the short circuit about said resistor todecrease the current supplied to said motor and responsiveto a decreasedvalue of said current'to reclose said short circuit to increasesaidcurrent to said predetermined value. I

3. In a motor control system, a motor for citation of said separatelyexcited Winding thereby to reduce the excitation of said generatorsubstantially to zero and responsive to substantially ZGIO current forincreasing the excitation of said separately. excited Winding so as toincrease'said current to said maximum permissible value. I v

4. In a variable voltage control system, a supply generator and a motorsupplied there from for driving a load, a separately excited fieldcircuit for said generator and meansior varying the excitation of saidcircuit for varying the voltage and current supplied to said motor, anoppositely Wound series field circuit for said generator for limitingthe current supplied to said motor to a maximum permissible value, aresistance in said separately excited field circuit, normally closedcontacts for short circuiting said resistance, an electroresponsivedevice responsive to the maximum permissible value of current suppliedto said motor for opening said contacts to interrupt the short circuitabout said resistance to decrease the current supplied to said motor andresponsive t'o'decreased current for closing said contacts to increasethe current supplied to said motor to said maximum permissible value.

CARL SCHIEBELER. ARNO VV'EISS.

